This invention relates to controlling the output of a laser and, more particularly, to a phase-locked loop circuit utilized to drive a pulse-operated laser.
Various ways of obtaining a uniformly spaced train of pulses from a continuously pumped laser are known. One of these techniques, designated cavity dumping, has been utilized, for example, to operate a Nd:YAG laser to achieve pulse repetition rates of from 125 kilohertz up to several megahertz. Cavity dumping of Nd:YAG lasers is described by R. B. Chesler and D. Maydan in Journal of Applied Physics, Vol. 42, No. 3, pages 1028-1034, Mar. 1, 1971.
Cavity-dumped Nd:YAG lasers are used in a number of applications of practical importance. One such application is a system for making reticles for integrated circuit fabrication. In this system the pulsed laser is utilized to selectively machine an iron oxide film to form a pattern composed of transparent and opaque regions.
To achieve high-quality reticles in a laser machining system it is essential that the output pulses provided by the laser be characterized by a high degree of amplitude stability and uniformity. To achieve this output condition it is necessary that the electrical control signals applied to drive the laser be exactly or nearly exactly spaced apart in time.
It has been found, however, that randomly occurring vibrations and noise in an actual machining system cause the control signals applied to the laser to deviate from perfect periodicity. Without some effective means for correcting this condition the machining system will not function to provide acceptable reticles.
Moreover, it has been found that the frequency of the control signals developed in the machining system for driving the laser is, as a practical matter, insufficiently high to meet various operating criteria established for the system.
Thus the need arose for a instrumentality that would be capable of processing the laser driving signals developed in the machining system to achieve a uniform spacing of these signals under actual working conditions. In addition, the need arose to multiply the frequency of these driving signals by a specified integral factor.